They're strategy games.
Civilization IV is obviously in the Civilization series, which is basically about building an empire. You research technologies, build cities and armies, and expand your empire's power. You can win via peaceful or warlike means. I like Civ IV best because it does better justice to religions, and is very easily moddable. It's easy to mod the .xml files if you're familiar with using HTML or UBB tags.
X-Com 2 is the sequel to the remake of X-Com. War of the Chosen is a major expansion pack to X-Com 2 which fills it out nicely. If you're interested primarily in story development and immersion, play X-Com first. That's the game where you command a global special forces unit to fight back against alien raids. X-Com 2 assumes you lost the campaign in X-Com, and the aliens took over. You must command a global insurgency against the aliens and the fascist regime they set up. It does have more layers to its strategy than X-Com (especially stealth, ambush, and carrying bodies). War of the Chosen adds more classes, more ways to use existing classes, covert ops missions, more "regular" missions, more enemies to fight, better music, and voice acting by several Star Trek: The Next Generation actors.